Magnetic resonance imaging apparatus and apparatus for measuring radio frequency output for the same

ABSTRACT

An apparatus for measuring radio frequency output for a magnetic resonance imaging apparatus includes a plurality of directional couplers, a comparator, a switcher and a converter. The plurality of directional couplers are different in degree of coupling from each other, and attenuate an RF signal which is generated in an RF signal generator and amplified in an RF power amplifier. The comparator compares input-level information of a signal inputted into the RF power amplifier with a threshold value. The switcher switches to any one of the plurality of the directional couplers based on a result of the comparison so as to output an RF signal by the one directional coupler. The converter performs a digital conversion of the RF signal from the one directional coupler so as to output a digital signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation Application of No. PCT/JP2013/75717,filed on Sep. 24, 2013, and the PCT application is based upon and claimsthe benefit of priority from Japanese Patent Application No.2012-211364, filed on Sep. 25, 2012, the entire contents of which areincorporated herein by reference.

FIELD

The present embodiments as an aspect of the present invention relate toa magnetic resonance imaging (MRI) apparatus and an apparatus formeasuring radio frequency output for the same.

BACKGROUND

MRI is an imaging method for magnetically exciting nuclear spin of anobject that is placed in a static magnetic field, with use of a radiofrequency (RF) pulse having the Larmor frequency and reconstructing animage from nuclear magnetic resonance signals generated with theexcitation. In MRI, an RF coil is used to transmit an RF pulse to animaging region to excite nuclear magnetic resonance. The resonantfrequency of the RF pulse is proportional to intensity of the staticmagnetic field of an MRI apparatus. For example, in the case of a staticmagnetic field of 1.5 tesla, the resonant frequency is 63.8 MHz.

In this frequency range, the RF pulse causes an increase in bodytemperature of the object. Accordingly, from the viewpoint of safety, anupper limit of the energy of the RF pulse transmitted to the object isprescribed by, for example, the International ElectrotechnicalCommission (IEC) standard or other standards. More specifically, energyof the RF pulse absorbed by 1 kg of living tissue is referred to as aspecific absorption ratio (SAR). It is prescribed that SAR values for,for example, arbitrary 10 seconds and for 6 minutes do not exceed afirst or second upper limit, respectively. The upper limit variesdepending on whether the imaging region is the entire body or a partialregion (such as the head).

In conventional technology, in order to satisfy the safety standardswith respect to the SAR, an integrated value of the energy of an RFpulse transmitted to the object is calculated for each of preceding 1second, 5 seconds, and 10 seconds. In any one of the following threecases, a pulse sequence is changed. A first case is that the integratedvalue exceeds a first predetermined value for the preceding one second.A second case is that the integrated value exceeds a secondpredetermined value for the preceding five seconds. A third case is thatthe integrated value exceeds a third predetermined value for thepreceding 10 seconds. In some cases, the pulse sequence is changed bystopping operation of an RF pulse generator. However, this may causeinterruption of imaging operation in the middle of the operation.

Accordingly, in the conventional technology, an SAR of the entire objectand/or a partial imaging region is calculated at the time of pre-scanbefore imaging. If the calculated partial SAR exceeds the upper limit,an alarm is displayed and then the pulse sequence is changed so that thepartial SAR does not exceed the upper limit. After it is verified that adose to the object does not exceed the upper limit of the partial SAR,imaging is performed.

In order to calculate the SAR, there are conventional technologieswhich, at the time of a pre-scan before a main scan, measure an energyvalue (or energy control value) of an RF signal attenuated by onedirectional coupler with a fixed degree of coupling, the attenuated RFsignal being based on an amplified RF signal from one RF power amplifierincluded in a transmitter. There are also conventional technologieswhich, at the time of the pre-scan before the main scan, measure anenergy value of an RF signal attenuated by a plurality of directionalcouplers arranged in series on a transmission line, the attenuated RFsignal being based on the amplified RF signal from the one RF poweramplifier.

However, in the case of the conventional technologies involving onedirectional coupler with the fixed degree of coupling, the accuracy ofan RF output monitor may deteriorate since MRI has a dynamic range witha wide transmission gain. While imaging of local regions, such as thelimbs, requires about 100 to 200 [W], imaging of the entire body (suchas imaging of an abdominal region) requires 10,000 to 20,000 [W]depending on the imaging sequence. The RF signal attenuated by thedirectional coupler is detected in a detector and is subjected to analogto digital (AD) conversion by an AD converter. Accordingly, in order tosupport a high power (10,000 to 20,000 [W]) signal, the degree ofcoupling of the directional coupler is set larger within a limit of amaximum input of the detector and the AD converter. In this case, alow-power signal (100 to 200 [W]) is excessively attenuated and itssignal level becomes susceptible to noise floor and offset, which causesa problem of deteriorated accuracy in detection and A/D conversion.

In the case of the conventional technologies involving a plurality ofthe directional couplers, the influence of a reflective RF signal may bereduced. However, since the degree of coupling is invariable, thelow-power signal (100 to 200 [W]) still suffers from the problem ofdeteriorated accuracy in detection and A/D conversion, as in theconventional technology involving one directional coupler.

BRIEF DESCRIPTION OF THE DRAWINGS

In accompanying drawings,

FIG. 1 is a schematic view showing a hardware configuration of an MRIapparatus according to a first embodiment;

FIG. 2 is a diagram showing a configuration of a transmitter in aconventional MRI apparatus;

FIG. 3 is a table view showing contents of attenuation and correction ofRF signals in the conventional MRI apparatus;

FIG. 4 is a diagram showing a configuration of a transmitter in the MRIapparatus according to the first embodiment;

FIG. 5 is a table view showing contents of attenuation and correction ofRF signals in the MRI apparatus according to the first embodiment;

FIG. 6 is a flow chart showing an operation of the MRI apparatusaccording to the first embodiment;

FIG. 7 is a schematic view showing a hardware configuration of an MRIapparatus according to a second embodiment;

FIG. 8 is a diagram showing a configuration of a transmitter in the MRIapparatus according to the second embodiment;

FIG. 9 is a table view showing contents of attenuation and correction ofRF signals in the MRI apparatus according to the second embodiment; and

FIG. 10 is a flow chart showing an operation of the MRI apparatusaccording to the second embodiment.

DETAILED DESCRIPTION

A magnetic resonance imaging (MRI) apparatus and an apparatus formeasuring radio frequency output for the MRI apparatus according to thepresent embodiments are described with reference to the accompanyingdrawings.

To solve the above-described problems, the present embodiments providethe apparatus for measuring radio frequency output for the MRIapparatus, including: a plurality of directional couplers different indegree of coupling from each other, and configured to attenuate a radiofrequency signal which is generated in a radio frequency signalgenerator and amplified in a radio frequency power amplifier; acomparator configured to compare input-level information of a signalinputted into the radio frequency power amplifier with a thresholdvalue; a switcher configured to switch to any one of the plurality ofthe directional couplers based on a result of the comparison so as tooutput a radio frequency signal by the one directional coupler; and aconverter configured to perform a digital conversion of the radiofrequency signal from the one directional coupler so as to output adigital signal.

To solve the above-described problems, the present embodiments providethe apparatus for measuring radio frequency output for the MRIapparatus, including: a directional coupler variable in degree ofcoupling, and configured to attenuate a radio frequency signal which isgenerated in a radio frequency signal generator and amplified in a radiofrequency power amplifier; a signal controller configured to control thedegree of coupling of the directional coupler; and a converterconfigured to perform a digital conversion of the radio frequency signalfrom the directional coupler so as to output a digital signal.

The apparatus for measuring RF output for the MRI apparatus according tothe present embodiments is able to accurately measure an RF output withsufficient precision even when the RF output is small.

To solve the above-described problems, the present embodiments providethe MRI apparatus, including: a static magnetic field magnet configuredto generate a static magnetic field; a gradient coil configured togenerate a gradient magnetic field where intensity of a magnetic fieldvaries, a transmission coil which is a radio frequency coil configuredto generate a radio frequency magnetic field; a radio frequency poweramplifier configured to amplify a radio frequency signal generated in aradio frequency signal generator and to provide the amplified radiofrequency signal to the transmission coil; a plurality of directionalcouplers different in degree of coupling from each other, and configuredto attenuate the radio frequency signal amplified in the radio frequencypower amplifier; a comparator configured to compare input-levelinformation of the signal inputted into the radio frequency poweramplifier with a threshold value; a switcher configured to switch to anyone of the plurality of the directional couplers based on a result ofthe comparison so as to output a radio frequency signal by the onedirectional coupler; a converter configured to perform a digitalconversion of the radio frequency signal from the one directionalcoupler so as to output a digital signal; and a calculator configured tocalculate a specific absorbed fraction based on the radio frequencysignal outputted from the converter.

The MRI apparatus according to the present embodiments is able toaccurately calculate an SAR with sufficient precision.

First Embodiment

FIG. 1 is a schematic view showing a hardware configuration of an MRIapparatus according to a first embodiment.

FIG. 1 illustrates an MRI apparatus 10 according to the first embodimentconfigured to image an object (patient) P. The MRI apparatus 10 ismainly made up of an imaging system 11 and a control system 12.

The imaging system 11 includes a static magnetic field magnet 21, agradient coil 22, a gradient power supply 23, a bed 24, a bed controller25, a transmission coil 26, a transmitter 27, reception coils 28 a to 28e, a receiver 29, and a sequencer (sequence controller) 30.

The static magnetic field magnet 21 is formed into a hollow cylindricalshape on an outermost portion of a mount (not shown) so as to generate auniform static magnetic field in an internal space. Examples of thestatic magnetic field magnet 21 include a permanent magnet and asuperconducting magnet.

The gradient coil 22 is formed into a hollow cylindrical shape and isarranged inside the static magnetic field magnet 21. The gradient coil22 is formed from a combination of an X-ch coil 22 x, a Y-ch coil 22 y,and a Z-ch coil 22 z each corresponding to X, Y, and Z axes which areorthogonal to each other. These three coils, 22 x, 22 y, and 22 z,individually receive current supply from the later-described gradientpower supply 23 and generate gradient magnetic fields where theintensities of the magnetic fields vary along each of the X, Y, and Zaxes. Note that a Z-axis direction is aligned with a direction of thestatic magnetic field.

The gradient magnetic fields in each of the X, Y, and Z axes generatedby the gradient coil 22 correspond to, for example, a gradient magneticfield Gr for readout, a gradient magnetic field Ge for phase encoding,and a gradient magnetic field Gs for slice selection, respectively. Thegradient magnetic field Gr for readout is used to change a frequency ofa nuclear magnetic resonance (NMR) signal in accordance with a spatiallocation. The gradient magnetic field Ge for phase encoding is used tochange a phase of an NMR signal in accordance with the spatial location.The gradient magnetic field Gs for slice selection is used toarbitrarily determine an imaging cross section.

The gradient power supply 23 supplies current to the gradient coil 22based on pulse sequence execution data sent from the sequencer 30.

The bed 24 includes a top plate 24 a to lay the object P thereon. Undercontrol of the later-described bed controller 25, the top plate 24 a ofthe bed 24 is inserted into a hollow (imaging port) of the gradient coil22 with the object P being laid thereon. The bed 24 is generally placedso that its longitudinal direction is parallel to a central axis of thestatic magnetic field magnet 21.

The bed controller 25 drives, under control of the sequencer 30, the bed24 so as to move the top plate 24 a in a longitudinal direction and in avertical direction.

The transmission coil 26 is arranged inside the gradient coil 22 togenerate a radio frequency (RF) magnetic field upon reception of an RFpulse from the transmitter 27.

The transmitter 27 transmits an RF pulse corresponding to the Larmorfrequency to the transmission coil 26 based on the pulse sequenceexecution data sent from the sequencer 30. The configuration of thetransmitter 27 is described later.

The reception coils 28 a to 28 e are arranged inside the gradient coil22 to receive an NMR signal emitted from the object P due to theinfluence of the RF magnetic field. The reception coils 28 a to 28 e arearray coils having a plurality of element coils which respectivelyreceive magnetic resonance signals emitted from the object P. Uponreception of the NMR signals with the respective element coils, thereceived NMR signals are outputted to the receiver 29.

The reception coil 28 a is a head-portion coil mounted on the head ofthe object P. The reception coils 28 b and 28 c are backbone coilsarranged between the back of the object P and the top plate 24 a. Thereception coils 28 d and 28 e are abdominal-portion coils each mountedon the abdominal side of the object P. The MRI apparatus 10 may includea coil for use in both transmission and reception.

The receiver 29 generates NMR signal data based on NMR signals which areoutputted from the reception coils 28 a to 28 e based on the pulsesequence execution data sent from the sequencer 30. Upon generation ofthe NMR signal data, the receiver 29 transmits the NMR signal data tothe control system 12 via the sequencer 30.

The receiver 29 has a plurality of receiving channels configured toreceive the NMR signals outputted from the plurality of the elementcoils included in the reception coils 28 a to 28 e. When an element coilto be used for imaging is notified from the control system 12, thereceiver 29 allocates a receiving channel to the notified element coilso as to receive an NMR signal outputted from the notified element coil.

The sequencer 30 is connected to the gradient power supply 23, the bedcontroller 25, the transmitter 27, the receiver 29, and the controlsystem 12. The sequencer 30 includes unshown processors such as acentral processing unit (CPU) and a memory. The sequencer 30 storessequence information describing control information necessary fordriving the gradient power supply 23, the bed controller 25, thetransmitter 27, and the receiver 29. The control information is, forexample, motion control information such as intensity, application time,and application timing of pulse current that should be applied to thegradient power supply 23.

The sequencer 30 also drives the bed controller 25 in accordance withthe stored specified sequence so as to move the top plate 24 a back andforth with respect to the mount in a Z direction. The sequencer 30further drives the gradient power supply 23, the transmitter 27, and thereceiver 29 in accordance with the stored specified sequence so as togenerate an X axis-gradient magnetic field Gx, a Y axis-gradientmagnetic field Gy, a Z axis-gradient magnetic field Gz, and an RF signalinside the mount.

The control system 12 performs control of an entire MRI apparatus 10,data collection, image reconstruction, and the like. The control system12 has an interface 31, a data collecting device 32, a data processingdevice 33, a storage 34, a display device 35, an input device 36, and acontroller 37.

The interface 31 is connected to the gradient power supply 23, the bedcontroller 25, the transmitter 27, and the receiver 29 of the imagingsystem 11 via the sequencer 30. The interface 31 controls input/outputof the signals delivered and received between each of these connectedunits and the control system 12.

The data collecting device 32 collects NMR signal data transmitted fromthe receiver 29 via the interface 31. Once the NMR signal data iscollected, the data collecting device 32 stores the collected NMR signaldata in the storage 34.

The data processing device 33 performs post-processing, i.e.,reconfiguration processing such as Fourier transform, on the NMR signaldata stored in the storage 34 so as to generate spectrum data or imagedata of desired nuclear spin in the object P. In the case of imaging apositioning image, the data processing device 33 generates, based on theNMR signals received in each of the plurality of the element coilsincluded in the reception coils 28 a to 28 e, profile data indicatingNMR signal distribution in an array direction of the element coils, foreach of the element coils. The data processing device 33 stores thegenerated various data in the storage 34.

The storage 34 stores the NMR signal data collected in the datacollecting device 32 and the image data generated in the data processingdevice 33 for each object P.

The display device 35 displays various kinds of information, includingthe spectrum data or image data generated in the data processing device33. Examples of the display device 35 include a display device, such asliquid crystal displays.

The input device 36 receives various operations and information inputsfrom an operator. As the input device 36, a pointing device such as amouse and a trackball, a selector device such as a mode selector switch,or an input device such as a keyboard, can suitably be used.

The controller 37, having an unshown central processing unit (CPU), amemory and the like, controls each of the units described in theforegoing for comprehensive control of the MRI apparatus 10.

FIG. 2 is a diagram showing a configuration of a transmitter in aconventional MRI apparatus.

A transmitter 57 in a conventional MRI apparatus 50 includes a referenceRF generator 61, a modulator 62, an RF power amplifier 63, a directionalcoupler 64, a wave detector 65, and an analog to digital (AD) converter66.

The reference RF generator 61 generates a reference RF signal (RFcarrier wave) under the control of the sequencer.

The modulator 62 modulates, under the control of the sequencer, thereference RF signal generated in the reference RF generator 61 into anRF signal with a specified waveform.

The RF power amplifier 63 amplifies the RF signal modulated in themodulator 62 and provides it to the transmission coil via thedirectional coupler 64. The amplified RF signal is transferred to thetransmission coil, and RF is emitted from the transmission coil to theobject. The transmission coil includes a transmission coil for wholebody and a transmission coil for local region.

The directional coupler 64 is arranged on a transmission line of the RFsignal in non-contact with the transmission line. The directionalcoupler 64 is configured to attenuate the RF signal, which istransferred to the transmission coil, with a required degree of coupling(coupling coefficient) and to send it to the wave detector 65. Thedirectional coupler 64 is a radio frequency device for attenuating theoutput (RF power) of the RF signal (a traveling wave and a reflectedwave). The output signal of the directional coupler 64 is detected bythe wave detector 65 in an MR signal processing substrate and isdigital-converted by the AD converter 66. The output data of the ADconverter 66 is used in order to calculate an SAR.

FIG. 3 is a table view showing contents of attenuation and correction ofthe RF signals in the conventional MRI apparatus.

A large degree of coupling is set for the directional coupler 64 withina limit of a maximum input of the wave detector 65 and/or the ADconverter 66 so that high-power (10,000 to 20,000 [W]) signals may besupported. For example, consider the case where the degree of couplingof the directional coupler 64 is 1/10,000. When a high-power (10,000[W]) RF signal is outputted from the RF power amplifier 63, a signal of1 [W] is inputted into the wave detector 65 via the directional coupler64.

When a low-power (100 [W]) RF signal is outputted from the RF poweramplifier 63, a signal of 0.01 [W] is inputted into the wave detector 65via the directional coupler 64.

The signals which are digital-processed by the AD converter 66 areoutputted to the sequencer, and the high-power and low-power RF signalsare respectively converted to 10,000 [W] and 100 [W] based on a 1/10,000degree of coupling of the directional coupler 64.

Since the degree of coupling of the directional coupler 64 is adjustedbased on the high-power signal, the low-power signal is excessivelyattenuated and an output of the low-power signal from the directionalcoupler 64 becomes as small as 0.01 [W]. Therefore, the signal level ofthe low-power signal is susceptible to the influence of noise floor andoffset, which causes the problem of deteriorated accuracy in detectionand A/D conversion.

FIG. 4 is a diagram showing a configuration of the transmitter 27 in theMRI apparatus 10 according to the first embodiment.

The transmitter 27 in the MRI apparatus 10 according to the firstembodiment includes a reference RF generator 41, a modulator 42, an RFpower amplifier 43, a directional coupler unit 44, a comparator 45, aswitcher 46, a wave detector 47, and an AD converter 48. The directionalcoupler unit 44, the comparator 45, the switcher 46, the wave detector47, and the AD converter 48 constitute an apparatus for measuring RFoutput in this embodiment.

The reference RF generator 41 generates a reference RF signal (RFcarrier wave) under the control of the sequencer 30.

The modulator 42 modulates, under the control of the sequencer 30, thereference RF signal generated in the reference RF generator 41 into anRF signal with a specified waveform.

The RF power amplifier 43 amplifies the RF signal modulated in themodulator 42 and provides it to the transmission coil 26 via thedirectional coupler unit 44. The amplified RF signal is transferred tothe transmission coil 26, and RF is emitted from the transmission coil26 to the object. The transmission coil 26 includes a transmission coilfor whole body and a transmission coil for local region.

The directional coupler unit 44 includes a plurality of directionalcouplers 44 a, 44 b, . . . different in degree of coupling from eachother. A description is hereinafter given of the directional couplerunit 44 including two directional couplers 44 a and 44 b. Thedirectional couplers 44 a and 44 b are arranged on a transmission lineof the RF signal in series in non-contact with the transmission line.The directional couplers 44 a and 44 b are configured to attenuate RFsignals, which are transferred to the transmission coil 26, withdifferent degrees of coupling, and to send attenuated the signals to theswitcher 46.

A signal is inputted from the modulator 42 to the RF power amplifier 43.The comparator 45 compares the input-level information (gaininformation) of the signal with a threshold value that is a referencesent from the sequencer 30. Based on the comparison output, thecomparator 45 controls the switcher 46. The comparator 45 controls theswitcher 46 which switches to any of the directional couplers 44 a and44 b. The output signal of the comparator 45 is used as a control signalof the switcher 46.

The RF signals obtained by attenuating RF signals in the directionalcouplers 44 a and 44 b are inputted into the switcher 46. The switcher46 switches to any one of the directional couplers 44 a and 44 b inresponse to the control signal of the comparator 45 so as to output anRF signal from the one directional coupler 44 a or 44 b to the wavedetector 47. The output signal of the switcher 46 is detected by thewave detector 47 in an MR signal processing substrate and isdigital-converted by the AD converter 48. In digital conversion of theoutput signal of the wave detector 47, the AD converter 48 multipliesthe output signal by a correction coefficient corresponding to theinput-level information of the signal inputted into the RF poweramplifier 43 from the modulator 42. The output data of the AD converter48 is transmitted to the control system 12 (shown in FIG. 1) via thesequencer 30, and the control system 12 uses the data to calculate anSAR.

FIG. 5 is a table view showing contents of attenuation and correction ofthe RF signals in the MRI apparatus 10 according to the firstembodiment.

A degree of coupling is set for the directional couplers 44 a and 44 bwithin a limit of a maximum input of the wave detector 47 and/or the ADconverter 48 so that high-power (10,000 to 20,000 [W]) signals may besupported. For example, consider the case where the degree of couplingof the directional coupler 44 a is 1/10,000 and the degree of couplingof the directional coupler 44 b is 1/100. When a high-power (10,000 [W])RF signal is outputted from the RF power amplifier 43, a signal of 1 [W]is inputted into the switcher 46 via the directional coupler 44 a, and asignal of 100 [W] is inputted into the switcher 46 via the directionalcoupler 44 b.

When a low-power (100 [W]) RF signal is outputted from the RF poweramplifier 43, a signal of 0.01 [W] is inputted into the switcher 46 viathe directional coupler 44 a, and a signal of 1 [W] is inputted into theswitcher 46 via the directional coupler 44 b.

When the RF signal of 10,000 [W] is outputted from the RF poweramplifiers 43, the switcher 46 adopts and outputs the signal of 1 [W]which is outputted from the directional coupler 44 a. When the RF signalof 100 [W] is outputted from the RF power amplifier 43, the switcher 46adopts and outputs the signal of 1 [W] which is outputted from thedirectional coupler 44 b.

The AD converter 48 converts an output value based on the degree ofcoupling that is a correction coefficient corresponding to theinput-level information from the modulator 42. More specifically, whenthe output signal of the directional coupler 44 a is adopted, the ADconverter 48 multiplies the output signal of the directional coupler 44a by 10,000 so as to convert the output signal into an output equivalentto 10,000 [W]. When the output signal of the directional coupler 44 b isadopted, the AD converter 48 multiplies the output signal of thedirectional coupler 44 b by 100 so as to convert the output signal intoan output equivalent to 100 [W] .

According to the aforementioned examples of the degree of coupling ofthe directional couplers 44 a and 44 b and/or the output of the RFsignals, the output of the switcher 46 results to be 1 [W] in both thecases of the high-power signal and the low-power signal. Therefore, thesignal level of both the high-power signal and the low-power signal isless susceptible to the influence of noise floor and offset, so thataccuracy in detection and A/D conversion is enhanced. It shouldnaturally be understood that the plurality of the directional couplers44 a and 44 b different in degree of coupling from each other may alsoenhance the accuracy in detection and A/D conversion in cases other thanthose described in the foregoing.

Next, the control system 12 shown in FIG. 1 controls the entire MRIapparatus 10 based on operation from an operator, while converting rawdata transmitted from the sequencer 30 into k space data andreconstructing an image based on the k space data. The control system 12also calculates an SAR of the entire object P and/or a partial imagingregion at the time of a pre-scan before a main scan (imaging). If thecalculated partial SAR exceeds the upper limit, an alarm is displayedand then the pulse sequence is changed so that the partial SAR does notexceed the upper limit. After it is verified that the dose to the objectdoes not exceed the upper limit of the partial SAR, the main scan isperformed. The pre-scan is a scan performed before the main scan, andits purpose includes at least adjustment of a transmission gain.

There has been described the process at the time of pre-scan in the MRIapparatus 10 according to the first embodiment. However, theabove-described process at the time of pre-scan is also applicable tothe process at the time of main scan. Now, the process of the main scanis described with reference to FIG. 4.

When an RF signal is transmitted from the transmission coil 26 tonuclear spin which is in a thermal equilibrium state due to the staticmagnetic field, the nuclear spin can be inclined (excited) by a certainangle. This angle is referred to as a “flip angle.” First, in thepre-scan performed before the main scan, an RF level, i.e., an RF signaloutput in the case of an excitation pulse having a reference flip angle(for example, 90 degrees) is measured. In general, the RF level variesdepending on load conditions of the transmission coil 26. For example,the measured RF value varies depending on the object P (depending on thebody thickness), and/or depending on positional relationship between thetransmission coil 26 and the object P (depending on the imaging regions)even when the object P is the same.

Next, based on the RF level, an RF signal output is set for each RFpulse (including a pre-pulse and an excitation pulsed) included in apulse sequence. The input-level information of a signal inputted intothe RF power amplifier 43 is controlled by automatic power control (APC)so that the set RF signal output is obtained. Typical pre-pulse examplesinclude fat suppressor pulses such as a short TI inversion recovery(STIR) pulse, a chemical shift selective (CHESS) pulse, a spectralpresaturation with inversion recovery (SPIR) pulse, and a spectralattenuated inversion recovery (SPAIR) pulse. The excitation pulse is apulse for inclining the nuclear spin by a flip angle.

As described in the foregoing, the RF signal output is set based on theRF level for each RF pulse included in the pulse sequence. Based on thisRF signal output, the input-level information of a signal inputted intothe RF power amplifier 43 in the main scan can be predicted for each RFpulse.

A description is now given of the operation of the MRI apparatus 10according to the first embodiment with reference to FIGS. 4 and 6.

FIG. 6 is a flow chart showing an operation of the MRI apparatus 10according to the first embodiment.

The MRI apparatus 10 sets patient information on the object P (such asweight and height) based on the information inputted by an operator onan imaging condition edit screen with the input device (shown in FIG. 1)(step ST1).

Based on the information inputted by the operator on the imagingcondition edit screen with the input device 36 (shown in FIG. 1), theMRI apparatus 10 sets an imaging region and imaging conditions (stepST2). The imaging conditions include a type of the pulse sequence(including the number of RF pulses), the number of multi-slices, and aslice thickness.

Next, the MRI apparatus 10 executes a pre-scan of the object P (stepST3). In the pre-scan in step ST3, the MRI apparatus 10 measures an RFlevel, i.e., an RF signal output in the case of the excitation pulsewith a reference flip angle (for example, 90 degrees), based on theimaging region set in step ST2 (step ST31). In the pre-scan in step ST3,the MRI apparatus 10 compares the input-level information of a signalinputted into the RF power amplifier 43 at the time of measurement instep ST31 with a threshold value. Accordingly, the directional couplers44 a and 44 b configured to attenuate the RF signal transferred to thetransmission coil 26 are switched (step ST32). The RF level variesdepending on the object P and the imaging region.

In step ST3, the MRI apparatus 10 detects an RF signal attenuated by thedirectional coupler 44 a or 44 b, and multiplies the RF signal by acorrection coefficient corresponding to the input-level information ofthe signal inputted into the RF power amplifier 43 so as to producedigital data. Based on the digital data, the MRI apparatus 10 calculatesSARs (entire SAR, partial SAR) and displays them on the display device35 (shown in FIG. 1) (step ST33).

Next, the MRI apparatus 10 transmits sequence information in conformitywith the imaging conditions set in step ST2 for the sequencer 30 toexecute a main scan of the object P (step ST4). In step ST4, the MRIapparatus 10 sets an RF signal output based on the RF level measured instep ST31 for each RF pulse (including a pre-pulse and an excitationpulse) included in the pulse sequence in conformity with the imagingconditions set in step ST2. Based on the RF signal output, theinput-level information of a signal inputted into the RF power amplifier43 in the main scan is predicted for each RF pulse (step ST41). In stepST4, the MRI apparatus 10 compares the input-level information predictedin step ST41 with the threshold value so as to switch between thedirectional couplers 44 a and 44 b which are configured to attenuate theRF signal transferred to the transmission coil 26 (step ST42).

In step ST4, the MRI apparatus 10 detects an RF signal attenuated by thedirectional coupler 44 a or 44 b, and multiplies the RF signal by acorrection coefficient corresponding to the input-level information ofthe signal inputted into the RF power amplifier 43 to produce digitaldata. Based on the digital data, the MRI apparatus 10 calculates SARs(entire SAR, partial SAR) and displays them on the display device 35(shown in FIG. 1) (step ST43). In step ST4, the MRI apparatus 10 alsoreconstructs an image (a two-dimensional image, a three-dimensionalimage) based on raw data collected in the main scan and displays theimage on the display device 35 (shown in FIG. 1) (step ST44). In stepST44, the SARs during main scan are monitored.

According to the transmitter 27 in the MRI apparatus 10 according to thefirst embodiment, the intensity of the RF signal to be detected andA/D-converted at the time of pre-scan and main scan is controlled. As aresult, even when an RF output is small, the RF output can accurately bemeasured with sufficient precision. Therefore, the MRI apparatus 10according to the first embodiment is able to accurately calculate theSAR with sufficient precision.

Second Embodiment

FIG. 7 is a schematic view showing a hardware configuration of an MRIapparatus according to a second embodiment.

FIG. 7 illustrates an MRI apparatus 10A according to the secondembodiment configured to image an object (patient) P. The MRI apparatus10A is mainly made up of an imaging system 11A and a control system 12.

The imaging system 11A includes a static magnetic field magnet 21, agradient coil 22, a gradient power supply 23, a bed 24, a bed controller25, a transmission coil 26, a transmitter 27A, reception coils 28 a to28 e, a receiver 29, and a sequencer 30.

Component members of the MRI apparatus 10A according to the secondembodiment shown in FIG. 7 which are identical to those of the MRIapparatus 10 according to the first embodiment shown in FIG. 1 aredesignated by identical reference numerals to omit description.

Like the transmitter 27 shown in FIG. 1, the transmitter 27A transmitsan RF pulse corresponding to the Larmor frequency to the transmissioncoil 26 based on pulse sequence execution data sent from the sequencer30.

FIG. 8 is a diagram showing a configuration of the transmitter 27A inthe MRI apparatus 10A according to the second embodiment.

The transmitter 27A in the MRI apparatus 10A according to the secondembodiment includes a reference RF generator 41, a modulator 42, an RFpower amplifier 43, a directional coupler 44A variable in degree ofcoupling, a wave detector 47, an AD converter 48, and a signalcontroller 49. The directional coupler 44A variable in degree ofcoupling, the wave detector 47, the AD converter 48, and the signalcontroller 49 constitute an apparatus for measuring RF output of thisembodiment.

Component members of the transmitter 27A shown in FIG. 8 which areidentical to those of the transmitter 27 shown in FIG. 4 are designatedby identical reference numerals to omit description.

When one directional coupler 44A variable in degree of coupling isprovided, the switcher 46 (shown in FIG. 4) becomes unnecessary, and theRF signal attenuated by the directional coupler 44A is directlyoutputted to the wave detector 47.

The directional coupler 44A is arranged on a transmission line of the RFsignal in non-contact with the transmission line. The directionalcoupler 44A is configured to attenuate the RF signal, which istransferred to the transmission coil 26, with a variable degree ofcoupling and to send it to the wave detector 47. The directional coupler44A is a radio frequency device for attenuating the electric power ofthe RF signal. The output signal of the directional coupler 44 isdetected by the wave detector 47 in an MR signal processing substrateand is digital-converted by the AD converter 48. In digital conversionof the output signal of the wave detector 47, the AD converter 48multiplies the output signal by a correction coefficient correspondingto the input-level information of the signal inputted from the modulator42 to the RF power amplifier 43 to produce data. The output data of theAD converter 48 is transmitted to the control system 12 (shown in FIG.7) via the sequencer 30, and the control system 12 uses the data tocalculate an SAR.

To control change in degree of coupling of the directional coupler 44A,the input-level information of the signal inputted from the modulator 42to the RF power amplifier 43 is inputted into the signal controller 49.The signal controller 49 changes the degree of coupling of thedirectional coupler 44A based on the input-level information.

FIG. 9 is a table view showing contents of attenuation and correction ofthe RF signals in the MRI apparatus 10A according to the secondembodiment.

An upper limit of the degree of coupling of the directional couplers 44Avariable in degree of coupling is set within a limit of a maximum inputof the wave detector 47 and/or the AD converter 48 so that high-power(10,000 to 20,000 [W]) signals may be supported. For example, considerthe case where the degree of coupling of the directional coupler 44Avariable in degree of coupling can be changed to 1/10,000. When ahigh-power (10,000 [W]) RF signal is outputted from the RF poweramplifier 43, a signal of 1 [W] is inputted into the wave detector 47via the directional coupler 44A.

A lower limit of the degree of coupling of the directional coupler 44Avariable in degree of coupling is also set so that low-power (100 [W])signals may be supported. For example, consider the case where thedegree of coupling of the directional coupler 44A variable in degree ofcoupling can be changed to 1/100. When a low-power (100 [W]) RF signalis outputted from the RF power amplifier 43, a signal of 1 [W] isinputted into the wave detector 47 via the directional coupler 44A.

The AD converter 48 converts the output value based on the degree ofcoupling that is a correction coefficient corresponding to theinput-level information from the modulator 42. More specifically, whenthe degree of coupling is 1/10,000, the AD converter 48 multiplies theoutput signal of the directional coupler 44A by 10,000 so as to convertthe output signal into an output equivalent to 10,000 [W]. When thedegree of coupling is 1/100, the AD converter 48 multiplies the outputsignal of the directional coupler 44A by 100 so as to convert the outputsignal into an output equivalent to 100 [W].

According to the aforementioned examples of the degree of coupling ofthe directional coupler 44A and/or the output of the RF signals, theoutput of the directional coupler 44A results to be 1 [W] in both thecases of the high-power signal and the low-power signal. Therefore, thesignal level of both the high-power signal and the low-power signal isless susceptible to the influence of noise floor and offset, so thataccuracy in detection and A/D conversion is enhanced. It shouldnaturally be understood that the directional coupler 44A variable indegree of coupling may also enhance the accuracy in detection and A/Dconversion in cases other than the above-described example.

Next, the control system 12 shown in FIG. 7 calculates SARs of theentire object P and/or a partial imaging region at the time of pre-scanbefore imaging. If the calculated partial SAR exceeds the upper limit,an alarm is displayed and then the pulse sequence is changed so that thepartial SAR does not exceed the upper limit. After it is verified thatthe dose to the object P does not exceed the upper limit of the partialSAR, the main scan is performed.

There has been described the process at the time of pre-scan in the MRIapparatus 10A according to the second embodiment. However, theaforementioned process at the time of pre-scan is also applicable to theprocess at the time of main scan, as described in the MRI apparatus 10according to the first embodiment.

A description is now given of the operation of the MRI apparatus 10Aaccording to the second embodiment with reference to FIGS. 8 and 10.

FIG. 10 is a flow chart showing an operation of the MRI apparatus 10Aaccording to the second embodiment.

In the flow chart of FIG. 10, the steps identical to those in the flowchart of FIG. 6 are designated by identical step numbers to omitdescription.

In the pre-scan in step ST3, the MRI apparatus 10A compares theinput-level information of a signal inputted into the RF power amplifier43 during measurement in step ST31 with a threshold value. Accordingly,the degree of coupling of the directional coupler 44A configured toattenuate the RF signal transferred to the transmission coil 26 iscontrolled (step ST32′).

In step ST4, the MRI apparatus 10A compares the input-level informationpredicted in step ST41 with the threshold value so as to control thedegree of coupling of the directional coupler 44A configured toattenuate the RF signal transferred to the transmission coil 26 (stepST42′).

According to the transmitter 27A of the MRI apparatus 10A according tothe second embodiment, the intensity of an RF signal to be detected andAD-converted during pre-scan and main scan is controlled. As a result,even when an RF output is small, the RF output can accurately bemeasured with sufficient precision. Therefore, the MRI apparatus 10Aaccording to the second embodiment is able to accurately calculate SARwith sufficient precision.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

What is claimed is:
 1. An apparatus for measuring radio frequency outputfor a magnetic resonance imaging apparatus, comprising: a plurality ofdirectional couplers different in degree of coupling from each other,and configured to attenuate a radio frequency signal which is generatedin a radio frequency signal generator and amplified in a radio frequencypower amplifier; a comparator configured to compare input-levelinformation of a signal inputted into the radio frequency poweramplifier with a threshold value; a switcher configured to switch to anyone of the plurality of the directional couplers based on a result ofthe comparison so as to output a radio frequency signal by the onedirectional coupler; and a converter configured to perform a digitalconversion of the radio frequency signal from the one directionalcoupler so as to output a digital signal.
 2. An apparatus for measuringradio frequency output for a magnetic resonance imaging apparatus,comprising: a directional coupler variable in degree of coupling, andconfigured to attenuate a radio frequency signal which is generated in aradio frequency signal generator and amplified in a radio frequencypower amplifier; a signal controller configured to control the degree ofcoupling of the directional coupler; and a converter configured toperform a digital conversion of the radio frequency signal from thedirectional coupler so as to output a digital signal.
 3. The apparatusfor measuring radio frequency output according to claim 2, wherein thesignal controller controls the degree of coupling of the directionalcoupler based on the input-level information of the signal inputted intothe radio frequency power amplifier.
 4. A magnetic resonance imagingapparatus, comprising: a static magnetic field magnet configured togenerate a static magnetic field; a gradient coil configured to generatea gradient magnetic field where intensity of a magnetic field varies, atransmission coil which is a radio frequency coil configured to generatea radio frequency magnetic field; a radio frequency power amplifierconfigured to amplify a radio frequency signal generated in a radiofrequency signal generator and to provide the amplified radio frequencysignal to the transmission coil; a plurality of directional couplersdifferent in degree of coupling from each other, and configured toattenuate the radio frequency signal amplified in the radio frequencypower amplifier; a comparator configured to compare input-levelinformation of the signal inputted into the radio frequency poweramplifier with a threshold value; a switcher configured to switch to anyone of the plurality of the directional couplers based on a result ofthe comparison so as to output a radio frequency signal by the onedirectional coupler; a converter configured to perform a digitalconversion of the radio frequency signal from the one directionalcoupler so as to output a digital signal; and a calculator configured tocalculate a specific absorbed fraction based on the radio frequencysignal outputted from the converter.
 5. The magnetic resonance imagingapparatus according to claim 4, wherein the input-level information ispredicted based on a radio frequency level which is a radio frequencysignal output in a case of an excitation pulse having a flip angle usedas a reference.
 6. The magnetic resonance imaging apparatus according toclaim 5, wherein the radio frequency level is measured in each objectand in each imaging region.
 7. The magnetic resonance imaging apparatusaccording to claim 5, wherein the input-level information is set foreach radio frequency pulse included in a pulse sequence based on theradio frequency level.
 8. The magnetic resonance imaging apparatusaccording to claim 7, wherein the radio frequency pulse included in thepulse sequence includes a pre-pulse and an excitation pulse.